it depends on what the stream is, flash with DRM seems out of the question.
easiest case would be, what you want to watch is reached via an rtsp:// URL.
that can be opened by VLC (hopefully any media player). VLC itself has streaming amenities.
but there, since you can read the stream without a browser then the pentium 4 can deal with it..
flashvideoreplacer is interesting, a firefox extension that purports to replace the youtube video player with a local video player (mplayer).
but crap, it's stopped! before I've tried it.
http://www.webgapps.org/addons/flashvideoreplacer#
you can try a software or browser extension that downloads the flash video. boring but an integrated solution which also lauch the video player is possible, I only don't know which it is (slashdot geeks who explain they don't need flash, or boast about watching streaming on their non x86 machine, know this)